This invention relates generally to methods and apparatus for magnetizing a permanent magnet, and specifically to magnetizing a magnet used in a magnetic resonance imaging (MRI) system.
There are various magnetic imaging systems which utilize permanent magnets. These systems include magnetic resonance imaging (MRI), magnetic resonance therapy (MRT) and nuclear magnetic resonance (NMR) systems. MRI systems are used to image a portion of a patient's body. MRT systems are generally smaller and are used to monitor the placement of a surgical instrument inside the patient's body. NMR systems are used to detect a signal from a material being imaged to determine the composition of the material.
These systems often utilize two or more permanent magnets directly attached to a support, frequently called a yoke. An imaging volume is providing between the magnets. A person or material is placed into an imaging volume and an image or signal is detected and then processed by a processor, such as a computer.
The prior art imaging systems also contain pole pieces and gradient coils adjacent to the imaging surface of the permanent magnets facing the imaging volume. The pole pieces are required to shape the magnetic field and to decrease or eliminate undesirable eddy currents which are created in the yoke and the imaging surface of the permanent magnets.
The permanent magnets used in the prior art imaging systems are frequently magnet assemblies or magnet bodies which consist of smaller permanent magnet blocks attached together by an adhesive. For example, the blocks are often square, rectangular or trapezoidal in shape. The permanent magnet body is assembled by attaching pre-magnetized blocks to each other with the adhesive. Great care is required in handling the magnetized blocks to avoid demagnetizing them. The assembled permanent magnet bodies comprising the permanent magnet blocks are then placed into an imaging system. For example, the permanent magnet bodies are attached to a yoke of an MRI system.
Since the permanent magnets are strongly attracted to iron, the permanent magnet bodies are attached to the yoke of the MRI system by a special robot or by sliding the permanent magnets along the portions of the yoke using a crank. If left unattached, the permanent magnets become flying missiles toward any iron object located nearby. Therefore, the standard manufacturing method of such imaging systems is complex and expensive because it requires a special robot and/or extreme precautions.
In order to magnetize the prior art permanent magnet, a pulsed magnetic field is used. The pulsed magnetic field is generated in a coil which is conventionally fabricated by layer winding rectangular wire. Because it is difficult to fabricate long lengths of large cross-section rectangular wires, numerous short lengths of wire are joined together to make the coil. These joints are frequently mechanically and electrically weak. Also, for thick wire winding, the layer to layer transition is difficult. These transitions often result in corner to corner contact which may damage insulation and result in a short during operation. Further, the transitions often result in a lower packing factor, losing a ¼ turn or more at the end of each layer.
An additional issue with the conventional pulsed magnetic coil is Joule heating from the pulse. Typically, the conventional pulsed coil is cooled in liquid nitrogen prior to applying the pulse to lower the resistivity of the copper coil. Below a temperature of 77 K, the resistivity of copper drops approximately eight fold. However, the passage of current during the pulse typically heats the coil above 77 K, resulting in a tremendous increase in resistivity. Therefore, in order to apply a second pulse, the coil must be remove from the precursor and cooled again.